The bottom surface of a sheet-like substrate (e.g., a glass sheet) can be contaminated in situ (i.e., while the substrate is inside a coater) due to overspray from a downward coating operation. For example, when material is sputtered downwardly onto the top surface of a substrate, some of the sputtered material can actually find its way onto the substrate's bottom surface. This overspray phenomenon has been found to leave unwanted coating on marginal portions of the substrate's bottom surface. This can create an undesirable “picture frame” effect on the substrate. Thus, when a downward coating operation is performed on a substrate, it would be desirable to provide means for cleaning (e.g., removing any unwanted oversprayed coating from) the substrate's bottom surface after the downward coating operation is performed. It would be particularly desirable to provide in situ means (i.e., means inside a coater) for cleaning the bottom surface of a substrate after its top surface has been coated by a downward coating operation.
Further, when a desired coating is applied to the bottom surface of a substrate by an upward coating operation after the top surface of the substrate has been coated by a downward coating operation, the marginal portions of the bottom surface can end up carrying both the desired coating and unwanted overspray from the downward coating operation, while the central portion of this surface carries only the desired coating. The resulting non-uniformity/picture frame effect, even if not discernable with the naked eye, is undesirable in that it can throw the coated substrate outside desired specifications. Thus, when a downward coating operation is performed prior to an upward coating operation, it would be particularly desirable to provide means for cleaning the bottom surface after the downward coating operation but before the upward coating operation.
Applying coating in an upward direction onto the bottom surface of a substrate can be highly advantageous. Upward coating operations can be advantageously performed in a coater in addition, or as an alternative, to downward coating operations. Particularly advantageous upward sputtering methods and equipment are described in U.S. patent application Ser. Nos. 09/868,542, 09/868,543, 09/979,314, 09/572,766, and 09/599,301, the entire contents of each of which are incorporated herein by reference. In conjunction with upward coating technology, it would be desirable to provide means for cleaning the bottom surface of a substrate before (preferably shortly before) such surface is coated by an upward coating operation. It would be particularly desirable to provide in situ means for cleaning the bottom surface of a substrate before such surface is coated by an upward coating operation.
When a substrate is transported through a coater using a substrate support, the substrate can be left with traces of contact from the support. Substrates are commonly conveyed through coaters using spaced-apart transport rollers, such that the bottom surface of each substrate is in direct contact with the rollers during conveyance. The rollers can leave traces of contact on the bottom surface of each substrate. While these traces of contact tend to be relatively minor (and completely acceptable for many applications), it would be desirable to provide means for assuring that any such traces of contact are completely removed from the bottom surface of the substrate before such surface is coated by an upward coating operation.
It would be particularly desirable to perform an ion beam treatment on the bottom surface of a substrate before applying a photocatalytic coating to such surface by an upward coating operation. This is done in certain embodiments of the present invention, for example, to facilitate depositing a high quality photocatalytic coating. This method is particularly preferred in embodiments wherein it is desired to deposit a particularly thin high quality photocatalytic coating.